The goal of our project is to investigate the causes of and evaluate potential novel therapies for the neurobehavioral symptoms associated with chronic hyperphenylalaninemia in phenylketonuria (PKU), one of the most common inborn errors of metabolism detected through newborn screening. Many adolescents and adults with PKU struggle with adherence to recommended dietary therapy. Chronic hyperphenylalaninemia is frequently associated with anxiety, depression, and impaired executive function such as difficulties with concentration and short-term memory. Deficiency of the monoamine neurotransmitters, dopamine and serotonin, in brain has been implicated as a probable proximal cause of neurobehavioral difficulties in PKU. The proposed causes of dopamine and serotonin deficiencies include decreased brain content of L-tyrosine and L-tryptophan, the substrates for dopamine and serotonin synthesis respectively, and phenylalanine- mediated competitive inhibition of tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH), the rate- limiting enzymes in the dopamine and serotonin synthetic pathways. Our overarching hypothesis is that restoration of brain dopamine and serotonin content will successfully ameliorate the neurobehavioral symptoms associated with chronic hyperphenylalaninemia. Our preliminary data document severe monoamine neurotransmitter deficiency in Pahenu2 mice, a model of human PKU, in association with a deficit in short-term memory as measured by a Morris Water Maze test. In our first specific aim, we will further define the behavioral phenotype of Pahenu2 mice and use molecular and biochemical methods to comprehensively evaluate amino acid and monoamine neurotransmitter metabolism in brain. The second aim of our project will focus upon evaluating the efficacy of several novel therapies in overcoming brain monoamine neurotransmitter deficiency and ameliorating behavioral abnormalities in the mice. The therapies to be evaluated individually and in combination will be large neutral amino acid (including L-tyrosine and L-tryptophan) supplementation to treat brain amino acid deficiencies, pharmacologic inhibition of tyrosine catabolism to raise brain tyrosine content, or administration of 5-hydroxytryptophan and L-DOPA to bypass TH and TPH and restore brain monoamine neurotransmitter synthesis. The outcomes will be compared to the effects of lowering blood and brain phenylalanine through dietary phenylalanine restriction or recombinant adeno-associated virus (rAAV)- mediated liver-directed gene therapy. In the final aim of the project, we will investigate, using microdialysis methods, whether acute administration of (6R)-5,6,7,8-tetrahydrobiopterin (BH4), the required cofactor for TH and TPH activity, can overcome phenylalanine-mediated competitive inhibition of these enzymes and restore dopamine and serotonin synthesis. Our goal is to thoroughly evaluate these novel therapeutic approaches in Pahenu2 mice and to choose specific interventions for a future clinical trial in humans with PKU.